Gentlemen,
I think there's some misunderstanding concerning feed line length and
proper matching. If the source and load are both the same impedance as the feed
line then you have a perfect 1:1 match and the impedance seen at the source is
the same no matter what the feed line length is. If there is a mismatch (load
and/or feel line impedance does not match the source impedance then there will
be a given SWR on the feed line. If for example there is a 2:1 purely resistive
mismatch (SWR) between the load and feed line then you will see a 2:1 SWR on
the entire feed line, and a 2:1 mismatch presented to the source no matter what
the feed line length. However, the impedance seen as one moves down the feed
line will go from 2:1 purely resistive, to 2:1 purely inductive (the resistive
component will match the feed line resistive impedance) to a 2:1 mismatch being
purely resistive but this time the inverse of the first case (if the load
resistive impedance was twice the feed line then now it will present an
impedance of half the feed line impedance) continuing on to a 2:1 mismatch
being purely capacitive (the resistive component is again the same as the feed
line's resistive impedance.
The only time a feed line impedance/length condition transforms a
mismatched load and source combo into a perfect match is when you have
situation where the line length and impedance are such that the impedance seen
looking into the feed line from the source's end happen to match the source's
impedance. There are some antenna/feed line combo designs which match a higher
antenna resistive impedance to a lower transceiver resistive impedance using a
feed line which has an intermediate resistive impedance and an exact length.
However this arrangement is very frequency specific/sensitive and the feed line
is still running less than perfect SWR with it's associated loss.
If you're using 50 Ohm antennas and 50 Ohm feed line and 50 Ohm
transmitters then feed line length is unimportant. If you are using
antennas/feedline/transmitters of differing impedances you'll have to use the
Smith chart and see what impedance is presented to the transmitter at varying
feed line lengths. Using a feed line of a specific length may or may not be
able to provide an adequate match depending on the impedance of the feed line.
If you're using coax you'll probably only have 50 Ohm cable and 75 Ohm cable to
work with unless you happen to live near a good electronic surplus dealer who
has scientific and/or military gear. Then you might have a few more options. Or
you can get copper tubing in various sizes and make your own custom feed line.
Some folks like using feed lines of certain lengths (in electrical
wavelengths) to try to avoid currents on the outside of the cable. That
technique was bigger before the advent of ferrous chokes.
Greg
On Jun 8, 2010, at 9:17 AM, Steve Barnes wrote:
> OK so I used the calculator and the length is 1.2113 Inches for 1/4 wave. I
> will already have 60 inches of cable. So what multiplier would be needed to
> keep the 1/4 wave between the antennas. Then the issue is how much cable is
> inside the feed horn?
> Sounds like screw them together and test to me?
>
> Steve Barnes
> RC-WiFi Wireless Internet Service
>
>
> -----Original Message-----
> From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org] On
> Behalf Of Mark Dueck
> Sent: Tuesday, June 08, 2010 9:06 AM
> To: wireless@wispa.org
> Subject: Re: [WISPA] Repeater
>
> I think something like this would help:
> http://www.csgnetwork.com/freqwavelengthcalc.html
>
>
> On 06/08/2010 06:32 AM, Steve Barnes wrote:
>> So with two 2.4 24 dbi Grid with 30" pig tail, what distance of cable would
>> you need in between them for the best match?
>>
>> Same with 5.8. Reason I ask is I have 2 locations that have no chance of
>> electrical power but need to get around a woods to. Both are very short
>> distances to tower but heavy woods for straight line of site. I actually
>> have some Grids lying around not being used that I could play with.
>>
>> Secondly I am a computer and network expert and I know what a wave is and
>> can measure one on an oscilloscope but have no idea how to convert that to
>> meter/feet/inch.
>>
>> Steve Barnes
>> RC-WiFi Wireless Internet Service
>>
>>
>> -----Original Message-----
>> From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org]
>> On Behalf Of Mike
>> Sent: Tuesday, June 08, 2010 12:03 AM
>> To: 'WISPA General List'
>> Subject: Re: [WISPA] Repeater
>>
>> You are absolutely right. A quarter wave feed line is an impedance inverter.
>> If my 1/4 wave multiples were even numbered the same effect would be found.
>> (2 x 1/4 wave = half wave) A half wave feed is an impedance repeater. I DO
>> find the idea intriguing, but not so that I will be the one to acid test it.
>> :-)
>>
>> Friendly Regards,
>>
>> Mike
>>
>> -----Original Message-----
>> From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org]
>> On Behalf Of Greg Ihnen
>> Sent: Monday, June 07, 2010 10:17 PM
>> To: WISPA General List
>> Subject: Re: [WISPA] Repeater
>>
>> My understanding is that a half wavelength long feed line presents
>> zero impedance transformation. See the Smith chart -
>> http://www.scott-inc.com/img/smith.gif
>>
>> The idea being that if you have an impedance of anything other than feed
>> line's impedance ("a perfect match" represented by the point marked 1.0 in
>> the center of the chart) and you plot that impedance it will be a certain
>> distance and direction from 1.0. Then using a compass you measure from 1.0
>> on the chart out to the plotted input impedance point and swing an arc and
>> draw a circle centered on 1.0 that intersects the plotted input impedance.
>> To calculate the impedance seen at any point along the feed line as
>> you move down the feed line's electrical length (fraction of a
>> wavelength) you move around the Smith chart (actually around the
>> circle you drew) and that will be the impedance seen at that point on
>> the feed line. The scale around the outer diameter of the Smith chart
>> reads in decimal fractions of a wavelength. If you go 1/4 wave down
>> the feed line that represents going .25 wavelength around the Smith
>> chart's outer scale which corresponds to going
>> 180 degrees around the chart (the point opposite of the input
>> impedance on the circle you drew). If you go a half wave down the feed
>> line then you go
>> 360 degrees around the circle that intersects the feed point impedance, in
>> other words you return to where you started.
>>
>> Greg
>> On Jun 7, 2010, at 9:59 PM, Mike wrote:
>>
>>
>>> The whole idea of a passive repeater intrigues me. Two times in many
>>>
>> years
>>
>>> I have done just that with limited success.
>>>
>>>
>>>
>>> The first was a metal building I built for Daystar Communications in
>>> SW Florida. It was our NOC and housed our customer support team as
>>> well as
>>>
>> the
>>
>>> techs. Cell phone coverage was the pits. What I did was point a
>>> Yagi at
>>>
>> a
>>
>>> known cell tower a few miles away. The feed line penetrated the
>>> building and fed a half wave dipole. One of the benefits of that
>>> particular time
>>>
>> in
>>
>>> my life is I had access to a very nice network analyzer. The dipole
>>> was
>>>
>> cut
>>
>>> very precisely, and the feedline, LMR 600 if I remember correctly,
>>> was cut to a multiple of ¼ wave and acted as an impedance repeater.
>>> In that way
>>>
>> any
>>
>>> matching errors to the feedline were negated. It gave cell phones in
>>> the building a couple bars and made usage possible.
>>>
>>>
>>>
>>> The second one was for a customer here in Iowa. They live down in a
>>> bowl and couldn't see my tower 2 miles away. They have a campground.
>>> Cell phones don't work well at all in the bowl. There is a pasture
>>> which has a hill that rises up from the bowl. From that hill you can see
>>> my tower.
>>> They planted a telephone pole and ran electricity to it. We put a
>>> panel pointed at my tower and a second one lower as a repeater which
>>> termed the entire property into a hot spot. It works well.
>>>
>>>
>>>
>>> We took 2 long commercial 800 MHz Yagis and connected them together
>>> with a short feedline measured, with the velocity factor to be a
>>> multiple of ¼
>>>
>> wave
>>
>>> again. One Yagi points at a cell tower, the other points at the
>>>
>> campground.
>>
>>> It gives cell phones a couple bars where they didn't work most of the
>>> time before.
>>>
>>>
>>>
>>> If you used a couple high gain, efficient dishes and separated them
>>> with minimum feedline or hardline, it should work in a similar way.
>>> I would be curious to see the results as I haven't done it with
>>> frequencies over 800 MHz. I wouldn't look for any magic results but
>>> reasonable results if your engineering is sound.
>>>
>>>
>>>
>>> Friendly Regards,
>>>
>>>
>>>
>>> Mike
>>>
>>>
>>>
>>> Mike Gilchrist
>>>
>>> Disruptive Technologist
>>>
>>> Advanced Wireless Express
>>>
>>> P.O. Box 255
>>>
>>> Toledo, IA 52342
>>>
>>> Mike's
>>>
>>>
>> <http://www.tamatoledonews.com/page/category.detail/nav/5001/Local-Col
>> umns.h
>>
>>> tml> Weekly Column
>>>
>>> 239.770.6203
>>>
>>> m...@aweiowa.com
>>>
>>>
>>>
>>> _____
>>>
>>> From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org]
>>> On Behalf Of Jack Unger
>>> Sent: Monday, June 07, 2010 5:21 PM
>>> To: WISPA General List
>>> Subject: Re: [WISPA] Repeater
>>>
>>>
>>>
>>> For this to work, one end needs to be very short. In a typical
>>>
>> mountain-top
>>
>>> repeat situation, the combined free-space path loss from BOTH paths
>>> is
>>>
>> more
>>
>>> than enough to prevent the link from working.
>>>
>>> Greg Ihnen wrote:
>>>
>>> Actually I've done this on ships where the deck department needed to
>>> communicate with the engine room and personnel down in compartments
>>> where winches for line handling were located. We're talking very
>>> short distances (less than the length of the ship - around a 1000 ft)
>>> and short cable
>>>
>> runs.
>>
>>> But it did let enough RF leak into the below deck areas to facilitate
>>> communications.
>>>
>>> Greg
>>>
>>> On Jun 7, 2010, at 4:34 PM, Bob Moldashel wrote:
>>>
>>>
>>>
>>> Garbage......
>>>
>>> Let's say optimum consideration here...
>>>
>>> Present RSL -68 db ....Subtract cable loss -2 dB = -70 Add +24 db
>>> for the Grid = -46
>>>
>>> Free Space Loss at 1/10th of a mile is -84 db
>>>
>>> Soooooo...
>>>
>>> If you take the -46 dB level out and add the FSL of -84 dB that will
>>> give you a -130 dB.
>>>
>>> I don't think that will work......
>>>
>>> Get a repeater or get a stronger receive signal at your receive
>>> antenna. Like -20dB
>>>
>>> -B-
>>>
>>>
>>>
>>>
>>> Steve Barnes wrote:
>>>
>>>
>>> Ok I have never even thought about doing this. Does it actually work?
>>>
>> This
>>
>>> sounds WAY to simple.
>>>
>>> A 29Db Grid on a Grain Leg pointed at the AP that has a -68 signal
>>>
>> plugged
>>
>>> into a 24 DB Grid Pointed to the house 1/4 mile away. What kind of
>>> signal would you have on the back side at the house?
>>>
>>> Steve Barnes
>>> RC-WiFi Wireless Internet Service
>>>
>>>
>>> -----Original Message-----
>>> From: wireless-boun...@wispa.org [mailto:wireless-boun...@wispa.org]
>>> On Behalf Of Rubens Kuhl
>>> Sent: Monday, June 07, 2010 4:20 PM
>>> To: WISPA General List
>>> Subject: Re: [WISPA] Repeater
>>>
>>> One option to consider is a passive repeater. Wire a coax cable
>>> between
>>>
>> the
>>
>>> two dishes and you are done... no electronics to fail, no power to
>>> supply
>>>
>> on
>>
>>> a remote location.
>>>
>>> (haven't tested this trick with dual polarity, though)
>>>
>>>
>>> Rubens
>>>
>>>
>>> On Mon, Jun 7, 2010 at 11:00 AM, Steve Barnes
>>> <mailto:st...@pcswin.com> <st...@pcswin.com> wrote:
>>>
>>>
>>>
>>> I have avoided repeaters like the plague but I have a situation where
>>> I
>>>
>> have
>>
>>> one and I am looking for a better option. When I started my wisp I
>>> was
>>>
>> 100%
>>
>>> Tranzeo. At this one location I setup a CPE connected to a TR-6000
>>> that
>>>
>> has
>>
>>> 2 Ethernet ports that pass through POE. I ran 1 Ethernet up the
>>> tower
>>>
>> with
>>
>>> a POE at the bottom, and a crossover in between.
>>>
>>> I would like a similar layout for other locations. Issue I see is that
>>>
>> not
>>
>>> many other units, UBNT or MT have a 2nd Ethernet that pass through POE?
>>>
>>> How does everyone you get around this?
>>>
>>> Trying to stay cheaper than a RB433, 2 radios, and 2- antennas, box,
>>> pigtails, 2 LMR cables.
>>>
>>> Steve Barnes
>>> RC-WiFi Wireless Internet Service
>>>
>>>
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>>>
>>>
>>> --
>>> Jack Unger - President, Ask-Wi.Com, Inc.
>>> Network Design - Technical Training - Technical Writing Serving the
>>> Broadband Wireless, Networking and Telecom Communities since
>>> 1993
>>> www.ask-wi.com 818-227-4220 jun...@ask-wi.com
>>>
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